Conventionally, a method of deterioration diagnosis for an insulating layer in a power cable for alternating-current (AC) power transmission has been variously studied. For example, in PTD 1, a deterioration diagnosis for an insulating layer is carried out concurrently using the residual charge method and the loss current method, both of which are applied in a cable in service using an AC voltage.
On the other hand, a technique for estimating a change in insulation performance of an insulator is disclosed, for example, in NPD 1. In NPD 1, when a direct-current (DC) voltage is applied to an insulator, the amount of electric charge accumulated inside the insulator is estimated to evaluate the insulation performance of the insulator.